1. Field of the Invention
The present invention relates to a method for the manufacture of a solar module comprising a front glass plate, a rear support plate and solar cells placed between these plates, the hollow space between these same plates being filled with a cast polyetherpolyurethane resin.
2. Discussion of the Background
Cast resin filled solar modules are known under various forms and are used in the building sector and the motor-vehicle sector in order to produce electric current from solar energy. In such uses, the solar cells are placed between two plates and coated with a layer of cast resin to protect them from atmospheric effects.
The cast resin must satisfy a variety of requirements. For example, like the front glass plate, it must possess a high transparency to light, especially in the near-infrared range, in order to enable the solar cells to be highly efficient. In addition, the cast resin must be insensitive to ultraviolet rays and to moisture. Finally, complete and lasting adhesion to the front glass plate and to the solar cells must be ensured. The cured resin must also have sufficient elasticity such that the mechanical stresses coming into play because of the differences in thermal expansion coefficients of the various materials are low in the polymer layer and cannot therefore cause deformation of the module or disbandment of the adhesive surfaces.
DE 2,712,172 A1 discloses the use of polycarbonates, silicones, epoxides, polyesters, acrylic resins and polymerizable products of chain molecules, with mainly covalently bonded hydrocarbons and no added halogen atom, such as polypropylene, polyisoprene or polybutadiene, as materials for encapsulating the solar cells.
DE 2,825,034 A1 proposes using a resin containing acrylic-acid components in order to coat the solar cells between the two plates. This type of resin may possess, as main component, a mixture of methacryl methacrylate and a polyurethane prepolymer.
On the other hand, DE 3,428,775 A1 recommends the use of EVA, EMA, acrylic resins, silicone elastomers, PVB, epoxy resins or aliphatic polyesterurethanes in order to manufacture solar modules possessing the aforementioned structure, in order to insert the solar cells between the two plates.
The requirements regarding the properties of the cast resin for producing solar modules become even more demanding as the size of the modules increases. While the conventionally used resins give results which are completely satisfactory with respect to the production of solar modules of small format, most of the known resins for this objective pose problems when used in manufacturing larger modules, as is especially the case in the building sector. Thus, when acrylates such as those employed in normal practice are used, the reduction in volume of the resin on being cured induces large stresses which, over time, cause disbondment between the resin and the solar cells.
It is possible to avoid such phenomena with acrylates only by casting highly plastic and highly flexible acrylate resins.
However, such resin layers have very low mechanical strength, so that the solar module cannot exhibit the properties of safety glass, properties which are, however, recommended for facade-type installations.
Other materials, such as silicones, are not suitable for large-format solar modules, also because of inadequate mechanical properties (especially because of a lower tear strength and poor solidity). The use of aliphatic polyester-urethanes has proved to be no longer satisfactory. Although it is true that polyesterurethane-based resins are also known to cure with virtually no reduction in volume, it has, however, been possible to demonstrate that polyesterpolyol-based polyurethanes progressively undergo weathering by hydrolysis. This is why solar modules which include an intermediate layer made of polyesterpolyurethanes have the drawback of not being able to be used without additional sealing of the edges. Without such sealing, it would be impossible to prevent the edge region of the module from being damaged in the long-term by moisture.